High temperature die lubricant



United States Patent Ofiflce 3,066,098 Patented Nov. 27, 1962 States ofAmerica as represented by the Secretary of the All Force N Drawing.Filed Apr. 8, 1960, Ser. No. 21,072 Claims. (Cl. 25225) This inventionrelates to improved die lubricants useful in forging metals.

Forged high stressed metal parts are being used in the construction ofaircraft. These parts very often include heavy metal sections joined tothin webs. In attempting to press forge such parts in metal dies greatdifliculty has been encountered in filling the die cavities in thelocation of the webs due to the rapid conduction of heat from the billetinto the die blocks and consequent chilling of the hot metal before thecavities are filled. This difficulty has been recently solved byelectrically heating the forging die-s to temperatures as high as 1900F. and forging the metal par-ts in the heated dies. By greatly reducingthe conduction of heat from the metal to be forged it is possible toform parts with thin sections without diflicul-ty from metals and metalalloys heretofore impossible to satisfactorily press forge insemifinished form, such as parts made from alloy steels, titanium,beryllium and their alloys. Further, it is possible by the use of heateddies to obtain much greater reduction of the stock without requiringreheat due to the lower conduction of heat from the stock to the dies.

The use of heated dies, however, gives rise to difficulty in releasingof the forgings from the dies due to sticking and pressure welding ofthe forging, surfaces to the die blocks causing the surfaces of the diecavities to be damaged and causing surface defects in the forgings.

In solving this problem it was determined that the dies must be coatedwith a lubricating or parting agent having low heat conductingproperties at high temperatures, have good adhering properties, benon-reactive with the die surfaces and have good lubricating andreleasing properties to prevent sticking of the forging to the diesurfaces. Preliminary tests with materials such as colloidal graphite,zinc oxide, lead oxide, molybdenum disulfide, molydenum oxide, powderedPyrex glass, glass wool, barium carbonate, powdered zircon, andmagnesium oxide indicated that high temperature refractory materialswould not serve as suitable parting agents, lead and lead oxide formedan adherent film on the heated dies at 1600 to 1800 F. and served as alubricant but were highly corrosive to the die surfaces at thesetemperatures. Flake graphite alone was somewhat satisfactory except forthe difliculty of coating overhead die surfaces.

In order to test the suitability of various lubricating materialselectrically heated dies were made from cast Inconel 713C alloy withflat polished engaging surfaces. The dies were heated with temperaturecontrol in steps from 600 to 2000 F. Cylindrical test pieces of A.I.S.I.4340 steel one-half inch in diameter and one-half inch high were heatedto 2250 F. and each successively inserted between the dies when thelatter were mounted in a hydraulic press and a constant load of 10,500lbs. applied. Tests were made at die temperatures of 600 F., 800 F.,1000 F., 1200 F., 1400 F., 1600 F., 1800 F. and 2000 F. At eachtemperature the dies were initially unlubricated and the test piecesubjected to load with the maximum reduction in height, the time toforge and the increase in diameter at the portions of the test piececontacting the die faces being carefully noted. As each lubricating orparting agent was tested the dies were coated with the material prior toinsertion of the test piece and the load then applied to the test piece.Where no lubricant was applied the reduction in height of the test pieceis primarily due to barreling or bulging of the sides with little or noincrease in diameter at the point of die contact while good lubricationwill allow the stock to expand radially, moving over the die faces,hence the increase in area of the portions of the test piece contactingthe die faces compared to the same area increase with no lubrication isa measure of the lubricating qualities of the par-ting material undertest. The comparative time required to forge at each die temperature isindicative of the insulating qualities of the parting material as isalso the percentage reduction in height.

From the tests on more than sixty individual mate rials and compositionsit was evident that no single material was suitable to meet all of thedesirable criteria.

It was found that the only satisfactory composition was a fusible metalsalt aoting in its molten state as a vehicle for a nonreactivelubricating component. The following alkali metal salts have been foundsatisfactory: potassium sulfate K 50 potassium bisulphate KHSO potassiumbisulphide KHS lithium chloride LiCl; and potassium iodide KI.

The fusible metal salt component may vary from a minimum of about 25% toa maximum of 60% by volume of the lubricating composition and the saltcomponent may be any of the above alone or mixtures thereof. Lithiumchloride While satisfactory would probably be ruled out because ofexpense and potassium sulphate K 50 is suitable only below dietemperatures of l800 F. since it is highly corrosive to the die surfacesat temperatures of 1900 F. and above. For use with die temperatures from600 F. to 1300 F. potassium sulphate, potassium bisulphate KHSO and mix-.tures thereof or either mixed in about equal parts by -volume withpotassium iodide are preferred. For die temperatures ranging from 1300F. to 1900 F. potassium iodide KI is preferred.

Suitable lubricating components of the composition have been found to bepowdered mica, powdered synthetic mica, powdered talc, powdered westernbentonite, and powdered flake graphite. The lubricating cgmponent of thelubricating composition may be employed singly or mixtures thereof maybe used. The total amount in percent by volume of the lubricatingcomponent or mixture of such components may vary from about 40% to byvolume of the lubricating composition.

Satisfactory formulations with all percentages indicated being percentby volume are as follows:

For use with die temperatures 600 F. to 1600 F.:

25% KHSO +25% bentonite+50% flake graphite 25% KI+25% bentonite+50%flake graphite 25 KI+50% -bentonite+25% flake graphite 25% KI+37.5%bentonite+37.5% talc 60% KI+20% talc+20% flake graphite 30% KI+30% K SO+20% bentonite+20% flake graphite 50% LiCl+50% flake graphite 30%LiCl+30% K SO +20% mica+20% flake graphite 50% KHSO +50% bentonite 33%Ki+67% flake graphite 50% KI+50% flake graphite For use with dietemperatures 1600 F. to 1800 F.:

50% KI+50% flake graphite 23% KI+67% flake graphite For use With dietemperatures KI+50% flake graphite Since die oxidation is greatlyaccelerated at temperatures above 1800 F. this is probably the maximumoperating temperature. The compositions having the best overallproperties for use in the lower die temperature range 600 F. to 1'300 F.is

25 KHSO +75% bentonite 50% KHSO +50% bentonite the latter formulationbeing preferred.

The formulation usable over the entire practical die temperature range600 F. to 1900 F. is: 33% KI+67% flake graphite.

The lubricating compositions in accordance with the invention are finelypowdered, for example powdered graphite (120 mesh), potassium iodide(140 mesh) and the powdered ingredients measured by volume andintimately mixed dry. The mixed formulation is applied to the heated diesurfaces by spraying the mixed dry powders with a compressed air spraygun with the spray orifices suitably enlarged. The alkali metal saltcomponent melts and tenaciously binds the lubricant component of themixture to the heated die surfaces.

While it is preferred to spray the lubricating composition in the drypowder form it is understood that the ingredients can be mixed with alight petroleum oil that will evaporate without leaving a carbon residueand applied with a spray gun.

The lubricating composition is applied to the heated dies so that itbuilds up to a coating of about twenty- .thousands of an inch thick, thecoating being initially very porous but which is reduced to aboutone-thouconsisting of a powdered fusible alkali metal salt nonreactiveto the heated die surfaces and selected from the group consisting ofpotassium sulfate, potassium bisulfate, potassium bisulphide, lithiumchloride, potassium iodide and mixtures thereof, and a finely dividedlubricant dispersed in said carrier, the lubricant being chemicallyinert with respect to the carrier and to the heated die surfaces andstable at forging temperatures, the lubricant being carried by themolten carrier into all parts of the dies in contact with the heatedforging stock, the lubricant being selected from the group consisting ofpowdered mica, talc, Western bentonite, and flaked graphite, the carriervarying from 25% to about 60% by volume of the lubricating composition.

2. A lubricating composition for lubricating forging dies consisting ofan intimate mixture of powdered potassium bisulfate and powderedbetonite, the potassium bisulfate being present in the range of from 25%to 50% by volume of the composition and the balance being bentonite.

3. A lubricating composition for lubricating forging dies consisting ofan intimate mixture of powdered potassium iodide and powdered bentonite,the potassium iodide being present in the amount of 25% by volume basedon the volume of the mixture from 25 to 50% by volume of bentonite andthe balance flake graphite.

4. A lubricating composition for application to press forging diesheated from 600 F. to 1900 F. consisting of an intimate mixture of 33percent by volume po tassium iodide and 67 percent by volume flakegraphite.

5. A lubricating composition for application to heated forging diesconsisting of an intimate mixture of powdered potassium iodide andpowdered flake graphite, the potassium iodide being present in an amountranging from about 20 percent to 50 percent by volume of the volume ofthe mixture, the balance being flake graphite.

References Cited in the file of this patent UNITED STATES PATENTS1,946,121 Wood Feb. 6, 1934 2,588,234 Hendricks Mar. 4, 1952 2,588,625Ferner ct a1 Mar. 11, 1952

1. IN THE ART OF PRESS FORGING OF METALS WHEREIN THE DIES AREELECTRICALLY HEATED TO TEMPERATURES IN THE RANGE OF FROM 600*F. TO1800*F. INDEPENDENT OF THE CONDUCTION OF HEAT THERETO BY THE HEATEDFORGING STOCK, THE METHOD OF LUBRICATING THE HEATED DIE CAVITIES TOINSURE FILLING THEREOF, READY RELEASE OF THE FORGING FROM THE DIES ANDHAVING NO DELETERIOUS EFFECTS ON THE DIE SURFACES CONSISTING OF COATINGTHE HEATED DIE CAVITIES WITH A LUBRICATING COMPOSITION, SAID LUBRICATINGCOMPOSITION CONSISTING OF A POWDERED FUSIBLE ALKALI METAL SALTNONREACTIVE TO THE HEATED DIE SURFACES AND SELECTED FROM THE GROUPCONSISTING OF POTASSIUM SULFATE, POTASSIUM BISULFATE, POTASSIUMBISULPHIDE, LITHIUM CHLORIDE, POTASSIUM IODIDE AND MIXTURES THEREOF, ANDA FINELY DIVIDED LUBRICANT DISPERSED IN SAID CARRIER, THE LUBRICANTBEING CHEMICALLY INERT WITH RESPECT TO THE CARRIER AND TO THE HEATED DIESURFACES AND STABLE AT FORGING TEMPERATURES, THE LUBRICANT BEING CARRIEDBY THE MOLTEN CARRIER INTO ALL PARTS OF THE DIES IN CONTACT WITH THEHEATED FORGING STOCK, THE LUBRICANT BEING SELECTED FROM THE GROUPCONSISTING OF POWDERED MICA, TALC, WESTERN BENTONITE, AND FLAKEDGRAPHITE, THE CARRIER VARYING FROM 25% TO ABOUT 60% BY VOLUME OF THELUBRICATING COMPOSITION.